Filter elements of this kind generally serve for filtration of gaseous media and are widely used as particulate air filters for removal of solid particulates such as dust, pollen etc. They may e.g. be placed in the outside air intake of a vehicles passenger compartment or may be used as combustion air filters to prevent abrasive particulate matter from entering an engine's cylinders, where the particulate matter would cause mechanical wear and possibly damage.
In practical applications, a filter element is usually placed in a filter casing, which, especially in mobile vehicles, offers very limited space. The efficiency of filter elements, on the other hand, strongly depends on the size of the surface area provided by the filter element for the passage of the medium to be filtered.
To improve filtration performance of such filter elements, the size of the surface of the filter element may be increased. With regard to the limited space provided by the respective casings, this can be achieved by an increased pleat height of the pleated filter medium.
However, when increasing the pleat height, pleat stability may be a problem when a certain pleat height is exceeded, the stability of the pleats depending also on the width of the filter element, i.e. on the length of the folding edges of the pleats. In other words, it is difficult to provide a filter element with has both a considerable pleat height and element width, as in such filter elements, the pleats tend to collapse due to their own weight and/or due to the flux of the gaseous medium that needs to be filtered. Thus, the passage for the medium to be filtered may become at least partially obstructed, increasing overall flow resistance and thereby decreasing efficiency of the filter element. As the stability of the pleats depends on the stiffness of the material of the filter medium used, this problem is particularly relevant when using filter media having a comparatively low stiffness, such as non-woven filter media.